FIELD OF THE INVENTION
The invention is in the field of multi-channel electrooptical transmitting devices for the parallel optical transmission of information. The invention relates to an electro-optical coupling module having a plurality of optical transmitters with an optically active zone in each case and a corresponding plurality of optical fiber ends that are assigned to one of the transmitters. The optical fiber ends each have an end face that receives radiation emitted from a respective optically active zone and deflects the radiation entering the optical fiber end radially.
Within the scope of the invention, an optical fiber is to be understood as any element for relaying an optical signal in a fashion guided with restriction in space, in particular manufactured optical fibers that are combined in a so-called optical fiber ribbon. A light-emitting region of the transmitter is to be understood as the optically active zone of the transmitter; in the case of a vertically emitting laser transmitter (so-called VCSEL), the optically active zone is located on the top side of the transmitting element.
A coupling module of the type mentioned at the beginning is known from German Patent DE 196 47 685 C1. In this coupling module, the ends of a plurality of optical fibers running in parallel are fixed in a common holder. An oblique end face of the holder is ground and polished together with the ends at a prescribed angle so that the end faces of the optical fibers act as reflection surfaces for light which is incident perpendicular to the optical fiber axis (radially). Light emitted by the optically active zones of individually drivable transmitters penetrates into the respective optical fiber end through the cladding thereof, and is deflected by the end face in the direction of the optical fiber longitudinal axis. Under normal conditions, the end surface acts in a totally reflecting fashion; with regard to unfavorable environmental conditions, the end face can also be silvered. The individual transmitters can be a constituent of a transmitting array.
In the case of the use of a plurality of individual transmitters, in particular in the case of the use of surface-emitting laser arrays (VCSEL), there are practical problems because of the relatively strong scattering of the individual response thresholds of the lasers. Moreover, the transmitters, in particular laser transmitters, have a comparatively high steepness. This has a problematic effect with respect to existing laser safety regulations, in accordance with which it is impermissible to exceed a prescribed limiting value of the output power of the individual transmitter. Specifically, since a uniform operating point usually has to be selected for all transmitters, the selection of a suitable operating point is extremely difficult in practice because of the scattering previously described. Specifically, while transmitters with a relatively low response threshold already output comparatively high powers for low control currents, other transmitters may not yet have reached their response threshold for these drive currents.
These problems could be countered in principle by undertaking a specific maladjustment between the individual transmitters and the respective assigned optical fiber end. However, this would have to be undertaken for each coupling module individually, and signifies a substantial outlay on measurement and production engineering. Moreover, if the path of radiation between the transmitter and optical fiber end is to be protected after the adjusting operation by an optically transparent potting, this method of maladjustment shows itself to be entirely unsuitable. Since, after its curing, the potting has a higher refractive index than the air (still present during the maladjusting operation), the optical relationships change after the curing. Any setting of the desired coupling efficiency which could be determined in advance would therefore be impossible.
It is, furthermore, conceivable to reduce the steepness of the transmitters, for example by suitable coatings. However, this necessarily results in additional heating of the transmitters, which seems to be unacceptable with regard to their service life and operational reliability.
Finally, it is conceivable for additives that influence the light transmitted through to be admixed to a material to be introduced between an optically active zone and optical fiber end. However, it is to be considered in this case that the distance between an optically active zone and optical fiber end is subject to configuration-induced tolerances that cause different influences of the light.